Bi-casting is a two-step process whereby one section of a component is cast, and then a second section is cast onto the first section in a second casting operation. Bi-casting has been utilized in gas turbine engine fabrication of vane rings and blades. Complex shapes can be designed for bi-casting that would exceed limits of castability in a single casting, and each section can have specialized material properties. Costly materials and processes such as single crystals can be selectively used where needed, reducing total cost.
A vane ring is a circular array of radially oriented stationary vane airfoils mounted between radially inner and outer shroud rings. The vane airfoils may be cast first, and then placed in a mold in which the inner and outer shroud rings are bi-cast onto the inner and outer ends of the airfoils respectively. The vane rings may be fabricated in segments. One or multiple vanes may be cast into an inner and/or an outer shroud segment to form a vane ring segment. A shroud segment on an end of a vane is called a platform.
A metallurgical bond may not form between the vane airfoils and the platforms. An oxide layer develops on the surface of the airfoil that prevents the molten metal of the platform from bonding to it. This may be overcome in order to form a bond. However, interlocking geometry without bonding has been used in the vane/platform interface to form a mechanical interconnection only.
In large gas turbines, differential thermal expansion (DTE) creates stresses between the vanes airfoils and shrouds. Providing clearance to accommodate DTE can result in lack of connection stability, stress concentrations, hot gas ingestion, and leakage of cooling air into the working gas flow from plenums and channels in the shrouds and vanes.